1. Field of the Invention
The present invention relates to a chromium-free metal surface treatment agent, which can replace a metal surface treatment that has been performed with a metal surface treatment agent containing chromate, in particular, chromate treatment of galvanized metal products, and the chromium-free metal surface treatment agent has excellent rust inhibitive performance compared with chromate treatment, and is able to prevent white rusting for a long time.
2. Description of the Related Art
The chromate treatment utilizes a metal-surface treatment agent containing chromic acid and has hitherto been used to improve the rust inhibitive performance of galvanized metal products. But because of the toxicity and carcinogenicity of hexavalent chromium component, the appearance of a chromium-free metal surface treatment agent has been strongly desired. For example, because the shredder dust of waste vehicles contains hexavalent chromium components derived from the chromate treated galvanized metal products, the waste disposal and recycling of the shredder dust are difficult.
If a chromium-free metal surface treatment agent having better performances than chromate treatment agents is excellent in cost performances, it is possible to avoid the chromate treatment and the chromium-free metal surface treatment agent is expected to contribute to the prevention of environmental pollution.
Although metal surface treatment agents containing a small amount of hexavalent chromium components and metal surface treatment agents utilizing a trivalent chromium component instead of a hexavalent chromium component are used at present, they are inferior in performances and the trivalent chromium can change to hexavalent chromium. Therefore, the use of these metal surface treatment agents cannot be said to be fundamental measures.
Several chromium-free metal surface treatment agents have already been proposed and some exhibit considerable performances. At present, they still have problems; for example, rust inhibitive performance equivalent to that obtained by chromate treatment cannot be obtained, they are not easy to use, white rusting is liable to occur, the self-repair function is poor, and the like.
For example, Japanese Laid-Open Patent JP 53-121034 A discloses metal surface treatment agents composed of a water reducible chromium-free aqueous solution of a silica complex. And, Japanese Laid-Open Patent JP 53-92846 A discloses a surface treatment method which comprises involves applying an aqueous solution of a silica complex formed from water dispersible silica, a water soluble or water dispersible acrylic copolymer and an alkoxysilane compound to a surface of a galvanized metal product.
Japanese Laid-Open Patent JP 10-46058 A mentions a metal surface treatment agent (water-based and solvent-based), which is mainly composed of silica such as silicates and colloidal silica, for surface treatment of steel products which are painted with a rust inhibitive paint containing a zinc powder as a rust inhibitive pigment. But, the examples of the Laid-Open Patent only include a water reducible metal surface treatment agent containing sodium silicate that is applied to a film of a rust inhibitive coating containing a zinc powder as a rust inhibitive pigment.
Japanese Laid-Open Patent JP 2000-355790 A discloses a chromium-free surface-treated galvanized steel sheet to which surface a galvanized layer having low impurity contents of Pb, Cu and Ag is formed by controlling the composition of a galvanizing bath, and a water reducible surface treatment solution containing silicates, colloidal silica, etc. is applied to the surface of galvanized layer, thereby improving white rusting resistance.
Japanese Laid-Open Patent JP 10-219138 A discloses a method of applying a top-coat layer which contains an inorganic filler, and an aqueous binder solution of silicates and/or colloidal silica, and the top-coat is formed on a surface of a ferrous substrate which is galvanized or painted by a zinc-rich paint. These water reducible surface treatment agents exhibits good performances to prevent the occurrence of red rusting, although there is the problem that white rusting is liable to occur.
Japanese Laid-Open Patent JP 2001-64782 A discloses rust inhibitive coatings in which oxide films of Si, Al or Ti are formed by the sol-gel method on hot-dip galvanized steel surfaces. In a silica coating described in the specification of the Laid-Open Patent, a galvanized steel plate is immersed in an alkoxide solution in a sol state (an alcoholic solution of an alkoxysilane oligomer which is obtained by mixing a small amount of water and hydrochloric acid to an alcoholic solution of tetraethoxysilane, and performing hydrolysis followed by condensation polymerization), pulled up, dried and baked, thereby forming the coating. However, the progress of condensation polymerization seems to be insufficient. White rusting occurred in 12 hours in the salt spray test described in the examples of the patent and the rust inhibitive performance was poor.
Furthermore, Japanese Laid-Open Patent JP 2001-232716 A discloses a corrosion resisting surface-treated galvanized steel sheet in which a base coating essentially formed from a silicate compound, such as an alkoxysilane compound including a silane coupling agent, a fluoride containing resin, such as fluorotitanic acid, and a phosphate compound. The base coating is formed on the galvanized steel sheet and a resin coating is formed as a top-layer.
Japanese Laid-Open Patent JP 2001-316845 A discloses a water reducible chromium-free metal surface treatment agent, the purpose of which is the rust inhibitive treatment of galvanized steels etc., which contains a silane coupling agent and/or its hydrolyzed condensation polymerization product, water dispersible silica, a zirconium compound and/or a titanium compound, a compound containing a thiocarbonyl group and/or a water soluble acrylic resin.
Japanese Laid-Open Patent JP 2002-129356 A discloses a water reducible coating composition effective in preventing the white rusting of hot-dip galvanized steels which consists essentially of a quaternary ammonium silicate, an inorganic filler, a synthetic resin emulsion and/or a water soluble synthetic resin, water and/or a hydrophilic organic solvent. In the examples of the Laid-Open Patent, there is described a composition in which a powder of pigment grade titanium dioxide having an average particle size of 0.5 μm is mixed. The weathering resistance of samples is investigated by use of a sunshine carbon arc type weather meter by applying the composition to commercially available hot-dip galvanized steel sheets.
Also, Japanese Laid-Open Patent JP 10-279885 A discloses a composition for a functional coating, the purpose of which is to impart various functions by application to surfaces of various types of materials, such as metals, ceramics, resins and wood, which consists of organoalkoxysilane or its hydrolyzed condensation polymerization product, a synthetic resin, a fine-grained metal oxide and/or carbon black, any one of a silver salt, a copper salt and colloidal silver, a hydrophilic organic solvent and water. Adhesion, impact resistance, good flexural properties, weathering resistance, heat resistance, chemical resistance, corrosion resistance, pollution resistance, electrical insulation, antibacterial properties, reducibility, liquid modifying properties, lipolytic properties, anticharging properties, etc. are enumerated as the various functions of the coating. In the examples in the Laid-Open Patent, a composition containing a nano-sized particle of titanium dioxide (titania) is mentioned and the use of a condensation polymerization product of organoalkoxysilane oligomer is suggested. However, the examples do not describe about a composition in which organoalkoxysilane oligomer is formed beforehand (it is explained that condensation polymerization is made after application). Also, although acid resistance and alkali resistance are investigated by adding an acid aqueous solution and an alkaline aqueous solution drop-wise from above, rust inhibitive performance which is usually evaluated by the salt spray test is not mentioned.